Set Appliance State:

Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Selector Switch ON
Gas Control Knob in Main Burner position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With the Main Burner alight, measure voltage across Modulation Coil (Grey wires).
The voltage should be in the range 9-13 Volts DC.